1. Field of the Invention
This invention relates to an image retransfer sheet through which an image once formed is retransferred to a secondary transfer medium by the action of heat and pressure, and more particularly to an image retransfer sheet through which a toner image formed using an electrophotographic apparatus, an electrostatic recording apparatus or the like is retransferred.
2. Related Background Art
Prior art relating to what is called decalcomania, a process by which images formed using an electrophotographic system are retransferred to a secondary transfer medium, is disclosed in, e.g., Japanese Patent Application Laid-open No. 52-82509. Image retransfer sheets used in this decalcomania are obtained by, e.g., forming on paper or plastic film on which a release layer comprised of a silicone resin or a fluororesine and having a high releasability has been formed an under coat layer comprised of a methyl methacrylate-n-butyl methacrylate copolymer, a polyvinyl acetate homogeneous polymer latex or a vinyl chloride-acrylate latex alone or in combination.
On the image retransfer sheet, toner images are formed using an electrophotographic apparatus or the like, and are fixed at least to such an extent that the toner images do not come off. After fixing, the image retransfer sheet is superposed on a secondary transfer medium (e.g., a cloth) face to face with the toner image side. In this state, they are heated under application of pressure to a temperature at which the toner and the under coat layer of the image retransfer sheet are softened, followed by cooling, and then the paper or the like having the release layer is peeled off in such a state that the toner images and the under coat layer are left on the secondary transfer medium side, thus the retransfer is completed.
The under coat layer used in this system is comprised of thermoplastic resin, where resins having a softening point equal or close to that of the toner resin are selected. Since the adhesion to the cloth or the like is attributable to the softening of this resin, practically the layer has a weak adhesive force and also a poor flexibility. In order to improve these points, the above Japanese Patent Application Laid-open No. 52-82509 discloses that a plasticizer is mixed in a thermoplastic resin to improve the flexibility of a resin film so that the re-fixing performance of toner resin on the cloth or the like may be ensured.
Mixing a plasticizer, however, tends to cause the toner resin to soften even after completion of retransfer, so that the images retransferred may adhere to any other article having come into touch with them, or retransfer-receiving surfaces may cling to each other if the secondary transfer medium is folded. In addition, in order to fully fix the toner images onto secondary transfer mediums such as cloth and wood, the former must be made to permeate into the latter under application of a considerably high pressure at the time of heating and pressing, and it is necessary to use a large-sized pressing machine for the exclusive use.
There is another method in which an aromatic hydrocarbon having a group containing a hydroxyl group is incorporated in the thermoplastic resin layer so that toner images may be retransferred to cloth or wood even under application of a low pressure. However, since the toner images may be transferred even when weakly heated and pressed, faulty paper transport may occur in electrophotographic apparatus.